The present invention relates to a hydraulic control system for an automatic transmission, and more particularly to an arrangement of shift valves in the hydraulic control system.
Commonly, in the conventional automatic transmissions, a plurality of shift valves are provided which change their states one after another to effect a shift between two speed ratios by supplying an oil pressure to appropriate clutch and/or brake. Usually, the supply of the oil pressure to an oil chamber of each of the friction elements and the discharge of the oil pressure thereof correspond to two states of one of the shift valves, respectively, i.e., the two states including an upshift state and a downshift state. The conventional automatic transmissions use a plurality of brakes, each being applied for establishing one of a plurality of forward speed ratios with reduction ratios. For example, a second brake is applied to establish a second speed ratio only and it is not applied during a first or a third speed ratio. As long as each of the brakes corresponds to one speed ratio, there occurs no problem even if the supply of oil pressure to an oil chamber and the discharge of oil pressure therefrom correspond to the two states of one shift valve, respectively. However, this design philosophy is not applicable to, for example a four speed automatic transmission with an overdrive wherein one brake is applied over more than two speed ratios because these speed ratios where the brake is to be applied cannot be distinguished. Therefore, the control of the four speed automatic transmission presents a problem that an additional shaft valve must be provided and in order to make a shift between the two speed ratios, two shift valves including the additional shift valve must shift simultaneously.